During the past few years there has been a substantial growth in the production of high-strength paper and cloth products having a nonwoven, randomly-oriented structure, bonded with a polymeric resin binder. Such products are finding wide use as high-strength, high-absorbency materials for disposable items such as consumer and industrial wipes or towels, diapers, surgical packs and gowns, industrial work clothing and feminine hygiene products. They are also used for durable products such as carpet and rug backings, apparel interlinings, automotive components and home furnishings, and for civil engineering materials such as road underlays. There are several ways to apply a binder to these materials, including spraying, print binding, and foam application. Further, depending on the end use, various ingredients such as catalysts, cross-linkers, surfactants, thickeners, dyes, and flame retardant salts may also be incorporated into the binder.
In the high-speed, high-volume manufacture of cellulosic products such as wet wipes, an important binder property is a fast cure rate; i.e., the finished product must reach substantially full tensile strength in a very short time after binder application so that production rates are not unduly slowed down. In these products, such a property is usually obtained by using a binder which is either self cross-linkable or by incorporating an external cross-linker into the binder formulation. When this is done, the cross-linker apparently not only interacts with the binder monomers but with the hydroxyl groups on the cellulose fibers to quickly form very strong bonds.
At present, there are a number of available binder formulations which meet this requirement. However, these materials are typified by incorporating one or more constituents which, over some period of time, will emit formaldehyde in amounts which may be sufficient to cause skin and respiratory irritation in many people, particularly children. Most recently, several of the leading manufacturers of nonwoven cellulosic products have expressed a desire to replace such binders with products offering equivalent levels of performance in cellulose but without the emission of formaldehyde. Although a number of ostensibly "zero" formaldehyde or "O CH.sub.2 O" cellulose binders have been proposed, they have either not been truly "O" in formaldehyde content or have not shown sufficiently fast cure rates to be acceptable in high-volume production applications.
One approach to a fast curing, "zero" formaldehyde binder for nonwoven cellulosic materials utilizes a binder comprising a solution copolymer formed by reacting a mixture of two or more water soluble olefinically unsaturated organic comonomers. The solution copolymer is admixed with a non-formaldehyde emitting latex to produce a final composition which is essentially free of formaldehyde and which, when cured on nonwoven cellulosic material, will achieve about 80 percent of fully cured wet tensile strength in 8 seconds or less.
While this approach results in providing zero formaldehyde emitting binders which are capable of fast curing, solution copolymers raise the viscosity and cause thickening of the binders in which they are incorporated. In certain applications, it is necessary to maintain the viscosity of the binder at a relatively low level in order to assure adequate penetration of the binder into the nonwoven substrate.